The present invention relates to a pneumatic radial ply runflat tire. More particularly, the present invention relates to an improved underlay, between the belts and the carcass of a radial ply runflat tire, that increases tire stiffness during runflat operation but not during normal inflated operation.
Various methods have been devised for enabling the safe continued operation of deflated or underinflated (flat) tires without damaging the tire further and without compromising vehicle handling while driving to where the tire can be changed. Loss of tire pressure can result from a variety of causes such as a deteriorated seal between the tire and rim or a tire puncture by a sharp object such as a nail.
Pneumatic tires designed for continued operation under deflated or underinflated conditions are referred to as xe2x80x9cextended mobility technologyxe2x80x9d tires or xe2x80x9cEMTxe2x80x9d tires. They are also called xe2x80x9crunflatxe2x80x9d tires, as they are capable of being driven in the flat condition. Runflat tires are designed to be driven in the deflated condition, whereas the conventional pneumatic tire""s sidewalls and tread buckle when subjected to a vehicle load while deflated. The sidewalls and internal surfaces of runflat tires do not collapse or buckle. In general, the terms xe2x80x9cEMTxe2x80x9d and xe2x80x9crunflatxe2x80x9d mean that the tire structure alone has sufficient strength to support the vehicle load when the tire is operated in the deflated state.
Numerous other methods and tire construction have been used to achieve runflat tire designs. For example, U.S. Pat. No. 4,111,249 discloses a runflat tire having an annular compression band (hoop), typically 15 centimeters wide, of solid high-strength metal or reinforced composite, located below the tread either under or embedded within the carcass. U.S. Pat. No. 4,059,138 discloses a runflat tire having, around the metal hub, an elastomeric ring that supports the inner central portion of the carcass when the tire is deflated.
Generally, runflat tires incorporate reinforced sidewalls that are sufficiently rigid so as not to collapse or buckle. Such sidewalls are thicker and stiffer than in conventional tires, so that the tire""s load can be carried by a deflated tire without compromising vehicle handling until the tire can be repaired or replaced. The methods of sidewall stiffening include the incorporation of wedge inserts (xe2x80x9cinsertsxe2x80x9d), which are fillers generally having a cross-sectional crescent shape. Such inserts are located in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire experiencing the greatest flex under load. In such runflat designs, the entire sidewall has an approximately uniform thickness corresponding to the thickness of the bead region, so as to provide runflat supporting rigidity. The sidewalls of such tires, when operated in the deflated condition, experience a net compressive load in which the outer portions of the sidewalls are under tension due to the bending stresses while the insides are correspondingly in compression, especially in the region of the sidewall midway between the tire""s bead region and the ground-contacting portion of the tread.
During runflat operation (i.e. while running underinflated), due to the large mass of rubber required to stiffen and reinforce the runflat tire""s sidewalls, heat buildup from cyclical flexure of the sidewalls is a major cause of tire failure, especially when the deflated tire is operated for prolonged periods of time and at high speeds. During normal inflated operation, the hysteresis of the material of the thickened runflat tire""s sidewalls contributes to its flexural heating, carcass fatigue, and rolling resistance, which reduces the vehicle""s fuel efficiency. The additional weight of the insert is also a disadvantage in handling and mounting a runflat tire.
In general, runflat tire design is based on the installation of one or more wedge inserts in each sidewall flex area. The wedge inserts, in combination with the ply structure, add rigidity to the sidewalls in the absence of air pressure during runflat operation. But this method has several drawbacks, including increased tire weight and heat buildup in the inserts, especially during runflat operation. Moreover, during runflat operation, bending stresses tend to be transmitted to the portion of the tread that contacts the ground, causing the central portions of the tread to tend to buckle upward from the ground, causing poor vehicle handling and reduced runflat tread life.
Bending stresses from the reinforced sidewall structures cause the footprint (portion of the tread containing the ground) to buckle upward in a meridionally bowed profile. Bending stresses from the tread portion adjacent to the footprint cause the footprint to buckle upward in a circumferentially bowed profile.
To reduce the aforementioned problems associated with stiffening the sidewalls with inserts, tire rigidity can be achieved by stiffening the tread with stiffening structural members under the tread. For example, U.S. Pat. Nos. 4,459,167 and 4,428,411 disclose runflat tires having an annular structural helical coil compression element on the inside surface of the carcass beneath the tread. This design stiffens the tire during runflat operation at the expense of stiffening the tire during normal inflation operation. PCT patent application PCT/US98/14054, filed Jul. 7, 1998, having a common assignee with the present invention, discloses a fabric underlay, between the belts and radial plies, that is reinforced by high-modulus cords that are parallel to the tire""s equatorial plane. The underlay stiffens the tread by widening the gap between the belts and plies. This design, too, stiffens the tire during runflat operation at the expense of stiffening the tire during normal inflation operation. U.S. Pat. No. 4,456,048 discloses a runflat tire having a xe2x80x9cbandxe2x80x9d (hoop) in the tire""s crown, whose shape exhibits dual-modulus of bending deflection. The shape can be either multiple xe2x80x9clandsxe2x80x9d (prisms) separated by xe2x80x9cslotsxe2x80x9d, xe2x80x9cV-shaped membersxe2x80x9d connected by xe2x80x9cannular fibersxe2x80x9d, xe2x80x9ccorrugated annular stripsxe2x80x9d reinforced with xe2x80x9cradial strutsxe2x80x9d, or xe2x80x9ca band element with an annular anticlastic shapexe2x80x9d. This design achieves the desired effect of stiffening the tire only in runflat operation (not in normal inflated operation), but this design is prohibitively costly to manufacture.
It is therefore desirable to have a runflat tire exhibiting significant rigidity during runflat operation but minimal rigidity during normal inflated operation. This would provide a softer ride during normal inflated operation and more rigid support during runflat operation. During both runflat and normal inflated operation, it would provide better handling and less rolling resistance, heat and tire degradation.
The present invention relates to a pneumatic radial ply runflat tire having a tread, a belt structure (xe2x80x9cbeltsxe2x80x9d) under the tread, a carcass, and an underlay between the belts and the carcass. The carcass has two inextensible annular beads, a radial ply structure and two sidewalls each reinforced with one or more wedge inserts. The tire is characterized by the underlay comprising a wound reinforcement cord disposed circumferentially under the belts with turns aligned parallel to the tire""s equatorial plane, the cord exhibiting dual modulus of elasticity (xe2x80x9cmodulusxe2x80x9d)xe2x80x94negligible (low or no) modulus below a threshold elongation and high modulus above the threshold elongation, and preferably exhibiting significant compressive modulus. A cord with such characteristics is also called a xe2x80x9c0 degree high elongation cordxe2x80x9d. The underlay may comprise one continuous cord spirally-wound to form the entire underlay, or may comprise a discontinuous cord (broken in places).
During normal inflated operation, the underlay does not stiffen the tire, because the layer has low modulus at low elongation, yielding a soft ride that is comparable to a similar tire without the underlay.
During runflat operation, the tread on either side of the footprint (section of tread contacting the ground) buckles outward, putting the tread""s outer surface under tension and inner surface under compression, with a xe2x80x9cneutral bending axisxe2x80x9d in-between. The underlay is in the inner, compression, side of the neutral bending axis, and so exhibits a significant compressive modulus, and hence stiffens the tread on either side of the footprint. The footprint buckles circumferentially upward, putting the footprint""s outer surface under compression and inner surface under tension, with a xe2x80x9cneutral bending axisxe2x80x9d in-between. The underlay is on the inner, tension, side of the neutral bending axis, and so exhibits high modulus, and hence stiffens the tread at the footprint.
In summary, the underlay stiffens the tire during runflat operation but not during normal inflated operation. During runflat operation, this reduces flexural wear and heat and improves ground contact. During normal inflated operation, this yields a soft ride.
Employing dual-modulus cords in the underlay simplifies manufacturing, rendering the underlay easier to stretch around the green carcass when the green carcass is expanded into engagement with the underlay.